Increased government regulation has reduced the allowable levels of particulates generated by diesel engines. The particulates can generally be characterized as a soot that is captured by particulate filters or traps. Present particulate filters or traps contain a separation medium with tiny pores that capture particles. As trapped material accumulates in the particulate trap, resistance to flow through the particulate trap increases, generating backpressure. The particulate trap must then be regenerated to burn off the particulates/soot in the particulate trap to reduce the backpressure and allow exhaust flow through the particulate trap. Past practices of regenerating a particulate trap utilized an energy source such as a burner or electric heater to generate combustion in the particulates. Particulate combustion in a diesel particulate trap by these past practices has been found to be difficult to control and may result in an excessive temperature rise.
Presently, conventional microwaves and microwave radiation are used in a variety of settings, including conventional microwave ovens. Heating by a microwave oven can be accomplished with a nonresonant cavity which is not designed with the purpose of exciting any particular microwave mode pattern. The field distribution within the nonresonant cavity will naturally exhibit standing waves, such that the microwave power absorption in a material exposed to the microwaves will be nonuniform. Analogous problems with using microwaves to heat a particulate trap in automotive applications also exist. Only portions of a microwave particulate trap may be heated when exposed to microwaves, leading to thermal runaway and less than satisfactory combustion of particulates in the particulate trap. This nonuniform heating can be minimized by the use of multiple microwave frequencies and/or mode-stirring using mechanical systems such as fan blades to cause a standing wave pattern to change in time in the cavity. Mechanical mode-stirring and the use of multiple microwave frequencies are not practical solutions in automotive microwave heating applications.